1. Field of the Invention
The present invention relates to a plasma thin-film forming apparatus for forming osmium thin-film on the surface of a film-coating body provided in the negative glow-phase zone.
2. Description of the Prior Arts
The present applicant has proposed a plasma thin-film forming apparatus in Japan Patent Application 1993-121595. This apparatus comprises a gastight vessel, a positive and a negative electrode disposed in the gastight vessel so that the discharge surfaces may face each other, an exhaust means for making a high vacuum condition in the interior of the gastight vessel, a gas-inducting means for diffusing material gas made from gasified predetermined metallic compound into the gastight vessel set in a high vacuum condition, and a high-voltage impressing means for generating DC glow discharge between the positive and the negative electrode. This apparatus may form a metallic thin-film by sticking and accumulating the positive-ionized gas molecules on the surface of a substrate disposed in the negative glow phase zone.
In this plasma thin-film forming apparatus, the gas-inducting means leads the material gas, which contains a metallic compound to be made into a thin-film on the surface of a thin-film-coating body, into the vacuum vessel and makes the gas pressure substantially constant in the vacuum vessel. In order to make the gas pressure constant, it was required to make constant concentration of the supplied material gas. Consequently, it is inevitable to accurately measure the crystalized metallic compound to be inducted in response to the required gasious concentration during forming the thin-film. Thus, the apparatus has a problem that the operation costs much labor.
Also in the apparatus, osmium tetraoxide is used as a material gas to form transparent thin-film that is low of electric resistant and superior in light transmission. However, osmium tetraoxide is very poisonous against the human body so that the working efficiency of forming thin-film was inferior because of difficulty to manage the gas to insure the safety of the operator. Especially, during the operation of forming thin-film, the constant gas pressure is maintained by discharging the exhaust air while osmium gas is supplied into the vacuum vessel; and after the operation of forming thin-film, the osmium gas is released into the atmosphere to recover the atmospheric pressure in the vacuum vessel. However, this was in danger of impairing the safety of the operator because of air contamination.
The present invention has been originated to resolve the above-described faults in the prior arts, and the object of the invention is to provide a plasma thin-film forming apparatus that is capable of forming a high accurate thin-film by simply metering a fixed quantity of the osmium gas to be inducted into the vacuum vessel.
Also, another object of the invention is to provide a plasma thin-film forming apparatus that is capable of securing the safety of the operator during the operation of forming thin-film.